In heterogeneous catalysis, what effect does increasing surface area have on the rate?

• A. Decreases rate
• B. Has no effect
• C. Changes only the mechanism
• D. Increases rate by providing more active sites

Answer: (D)

In heterogeneous catalysis the reaction takes place on the surface of a solid catalyst, so the overall rate is set by how many active sites are available for the reaction. Increasing the surface area exposes more active sites, allowing more reactant molecules to adsorb and react at once. Because each site follows the same chemistry, the rate per site doesn’t necessarily change; you’re simply multiplying the number of sites by giving the surface more area. Under typical conditions (when mass transport isn’t limiting), this leads to a higher overall rate, roughly proportional to the increase in active sites. If diffusion to the surface becomes the bottleneck, the increase may be less dramatic, but the fundamental effect is that more surface area provides more opportunities for reaction rather than altering the mechanism.